Liquid crystals have found use in a variety of electro-optical applications, in particular those requiring compact, energy efficient, voltage-controlled light valves such as watch and calculator displays. These devices are based on dielectric alignment effects in nematic, cholesteric, and smectic phases in which, by virtue of the dielectric anisotropy, the average molecular long axis takes up a preferred orientation in an applied electric field. Since the coupling of an applied electric field by this mechanism is rather weak, the electro-optical response time for these devices is too slow for many potential applications.
Liquid crystal devices (LCD) have a number of unique characteristics including low voltage and low power operation making them perhaps the most promising of the non-emissive electro-optic display candidates. The slow response and insufficient non-linearity in LCD's have been the serious limitations. The lack of speed becomes especially important in proportion to the number of elements that have to be addressed in a device. This leads to increasingly impractical production costs for flat-panel displays with potential use in computer terminals, oscilloscopes, radar and T.V. screens.
FIG. 1 is a schematic of a chiral smectic C or H liquid crystal. Crystal 100 is comprised of layers 102 of molecules. Within each layer 102, the average orientation of the long axes, denoted by the unit vector n, is tilted at an angle .PSI..sub.O to the layer normal. Meyer et al, in an article entitled "Ferroelectric Liquid Crystals" in Le Journal de Physique, Volume 36 (March, 1975, pp. L-69 to L-71) show that smectic C or H liquid crystals made of optically active molecules (chiral smectic C or H liquid crystals) would, in general, be ferroelectric, possessing an electric dipole density, P, which is perpendicular to the molecular tilt direction, n, and parallel to the smectic layer planes. Their demonstration also applies to the smectic H phase, which is close in structure to the smectic C, but exhibits a higher viscosity against the reorientation of n about an axis normal to the layers. The presence of the electric dipole in these chiral smectics provides a much stronger coupling of the molecular orientation to the applied electric field, E, than is available via the dielectric anisotropy. Furthermore, the coupling is polar in that the preferred orientation of P is parallel to E so that reversing the polarity of the applied electric field reverses the preferred orientation of P, meaning that field reversal can be effectively used to control molecular orientation.
Two properties of ferroelectric smectic liquid crystals have impeded the development of convenient methods of exploiting their intrinsic advantages. First, in ferroelectric smectics, the polarization must lie parallel to the layered planes as in FIG. 1 but must be otherwise unconstrained. Consequently, unlike crystalline ferroelectrics, the ferroelectric smectic liquid crystal does not spontaneously form domains of uniform orientation of P in the bulk, as this requires preferred orientation directions which, in a crystalline ferroelectric, for example, are provided by the crystal lattice.
Second, as an additional result of the molecule chirality, in a bulk ferroelectric smectic C or H liquid crystal, the unit vector n and polarization P spiral about the axis normal to the layers from layer to layer through the sample as in FIG. 1. The spiralling cancels the macroscopic dipole moment and corresponds to macroscopic cancelling of polarization by domain formation in crystalline ferroelectrics.
Thus, in FIG. 1, cone 104 for each layer 102 represents the locus of possible orientations of the molecules in a layer. Note that the cone forms an angle .PSI..sub.O with its longitudinal axis. Unit vectors (n) 106 through 130 represent the average alignment of the long axes of the molecules in each of the layers. As can be seen, the projection of vector 106 onto the plane represented by the sheet of drawing is a line normal to the plane of layers 102. Moving down through the layers, the average unit vector n of the molecules spiral. Thus, the unit vectors spiral through an azimuthal angle .phi., indicated in FIG. 1 of .phi.=90.degree. between vector 106 and vector 112. The unit vectors spiral through 180.degree. between vector 112 and vector 124. Finally the unit vectors spiral another 90.degree. between vector 124 and vector 130, so that the total spiral from vector 106 to vector 130 is 360.degree.. Thus, the average unit vector n of molecules in the layer associated with unit vector 106 is parallel with the average unit vector n of molecules in the layer associated with unit vector 130. The distance between the layer having unit vector 106 and the layer having unit vector 130 is referred to as the pitch of the helix formed by molecules in a direction perpendicular to the layers. In each layer the ferroelectric polarization P is normal to n and lying in the layer plane.
The distortion and unwinding of the spiral by an applied electric field, similar to that observed in chiral nematics, has been demonstrated. See Meyer et al, op. cit., and Martinot-Legarde, "Observation of Ferroelectrical Monodomains in the Chiral Smectic C Liquid Crystals", Le Journal de Physique Colloq, Volume 37 (1976, pp. C3-129 through C3-132), Martinot-Legarde, "Direct Electrical Measurement of the Primitive Polarization of a Ferroelectrics Chiral Smectic C Liquid Crystal", Le Journal de Physique, Volume 38 (January, 1977, pp. L-17 through L-19), and Takezoe et al "Birefringence in the Sm A Phase and the Disappearance of Helicoidal Structure in the Sm C Phase Caused by an Electric Field in DOBAMBC", Japanese Journal of Applied Physics, Volume 17, No. 7 (July, 1978, pp. 1219-1224). The suppression of the helix in chiral smectic C liquid crystals by boundary conditions which require molecules near the boundary to be parallel to the surface and in a particular direction has also been observed. See Brunet et al "Defauts dans les Smectiques C Chiraux", Ann. Phys., Vol. 3, No. 2-3-4 (1978, pp. 237-247).
Although the advantages of ferroelectric liquid crystals to produce electro-optic effects has been recognized, such effects have not been demonstrated. See Meyer, "Ferroelectric Liquid Crystals; A Review", Mol. Cryst. Liq. Cryst., Volume 40 (1977, pp. 33-48, and especially pp. 36, 38 through 40, 47). On the contrary, although several research groups in different countries have, since 1975, investigated the response of ferroelectric liquid crystals to electric fields, they have not found a significantly faster response than that characteristic of already existing kinds of liquid crystals. It would thus be highly desirable to provide a method whereby the strong coupling of molecular orientation to the applied electric field available in ferroelectric liquid crystals could be effectively utilized to provide a fast, polarity sensitive electro-optical device.